Capacitors are storage devices that store electrical energy on an electrode surface. Electrochemical cells create an electrical charge at electrodes by chemical reaction. The ability to store or create electrical charge is a function of electrode surface area in both applications. Ultracapacitors, sometimes referred to as double layer capacitors, are a third type of storage device. An ultracapacitor creates and stores energy by microscopic charge separation at an electrical chemical interface between electrode and electrolyte.
Ultracapacitors are able to store more energy per volume than traditional capacitors and they typically deliver the energy at a higher power rating than many rechargeable batteries. Ultracapacitors comprise two porous electrodes that are isolated from electrical contact by a porous separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Each electrode is in intimate contact with a current collector. One purpose of the current collector is to reduce ohmic loss. If the current collectors are nonporous, they can also be used as part of the capacitor case and seal.
When electric potential is applied to an ultracapacitor cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface, the ionic charge accumulates to create a charge layer at the solid liquid interface region. This is accomplished by adsorption of the charge species themselves and by realignment of dipoles of the solvent molecule. The adsorbed charge is held in this region by opposite charges in the solid electrode to generate an electrode potential. This potential increases in a generally linear fashion with the quantity of charge species or ions stored on the electrode surfaces. During discharge, the electrode potential or voltage that exists across the ultracapacitor electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode while an electronic current flows through an external circuit between electrode current collectors.
In summary, the ultracapacitor stores energy by separation of positive and negative charges at the interface between electrode and electrolyte. An electrical double layer at this location consists of sorbed ions on the electrode as well as solvated ions. Proximity between the electrodes and solvated ions is limited by a separation sheath to create positive and negative charges separated by a distance which produces a true capacitance in the electrical sense.
During use, an ultracapacitor cell is discharged by connecting the electrical connectors to an electrical device such as a portable radio, an electric motor, light emitting diode or other electrical device. The ultracapacitor is not a primary cell but can be recharged. The process of charging and discharging may be repeated over and over. For example, after discharging an ultracapacitor by powering an electrical device, the ultracapacitor can be recharged by supplying potential to the connectors.
The physical processes involved in energy storage in an ultracapacitor are distinctly different from the electrochemical oxidation/reduction processes responsible for charge storage in batteries. Further unlike parallel plate capacitors, ultracapacitors store charge at an atomic level between electrode and electrolyte. The double layer charge storage mechanism of an ultracapacitor is highly efficient and can produce high specific capacitance, up to several hundred Farads per cubic centimeter.
The presence of water in a nonaqueous ultracapacitor has a detrimental effect on device performance. The water limits both service life and energy density. Water causes corrosion of current collectors and limits service life. The electrochemical decomposition of water occurs at a voltage well below organic electrolyte breakdown. Hence, the presence of water limits operating voltage of a nonaqueous ultracapacitor. Limiting the operating voltage greatly affects stored energy since it increases as the square of voltage to which charge is taken. Finally, electrochemical decomposition of water into oxygen and hydrogen results in a gas accumulation within ultracapacitor devices. Accumulation of gases may breach the ultracapacitor enclosure to cause electrolyte leakage and may displace electrolyte from pores in the electrode and separator to increase resistance.
Water in a nonaqueous ultracapacitor can be reduced by a number of methods. The first method is to pre dry all materialsxe2x80x94electrolyte, separator and activated carbon, etc. Once the materials are dry, they are assembled and sealed in a water-free environment. However this procedure is expensive and rarely achieves complete water removal.
It is known that water electrochemically decomposes into oxygen and hydrogen at a voltage much lower than the operating voltage of a nonaqueous ultracapacitor. The present invention is directed to an electrochemical process to decomposes water in nonaqueous ultracapacitors into oxygen and hydrogen and then to remove the oxygen and hydrogen under vacuum. The process is inexpensive and effective in removing water from the structure.
The invention relates to a method of making an ultracapacitor. In the method, an ultracapacitor cell is electrolyzed prior to sealing the cell to form the ultracapacitor.
In another aspect of the invention, a multilayer cell is provided that comprises two solid, nonporous current collectors, two porous electrodes separating the current collectors, a porous separator between the electrodes and an electrolyte occupying pores in the electrodes and separator. The mutilayer cell is electrolyzed to disassociate water within the cell to oxygen gas and hydrogen gas. A vacuum is applied to the cell substantially at the same time as the electrolyzing step, to remove the oxygen gas and hydrogen gas. The cell is then sealed to form a ultracapacitor substantially free from water.
In another aspect, the invention relates to a method of making a stacked ultracapacitor. In the method, a stack of multilayer cells is provided wherein each cell comprises two solid, nonporous current collectors, two porous electrodes separating the current collectors, a porous separator between the electrodes and an electrolyte occupying pores in the electrodes and separator. The stack is electrolyzed to disassociate water within the cell to oxygen gas and hydrogen gas. A vacuum is applied to the stack substantially at the same time as the electrolyzing step, to remove the oxygen gas and hydrogen gas. The stack is then sealed to form a stacked ultracapacitor substantially free from water.